The evolution of Israel as a nation into Judaism as a religion was paralleled by a move from assimilation of strangers to a more formal idea of conversion.

The Biblical Israelites had no concept of religious conversion because the notion of a religion as separate from a nationality was incoherent.
The words "Jews" and "Judaism" did not exist. Abraham was called an ivri, a Hebrew, and his descendants were known either as Hebrews, Israelites (the children of Israel), or Judeans. These words are nationalistic terms that also imply the worship of the God of Abraham.

Earliest Form of "Conversion" was Assimilation

While there were no "conversions," many non-Israelites joined the Israelite community, often through marriage or acceptance of the beliefs and practices of the community. In this sense, assimilation is the earliest form of conversion. Abraham and his descendants absorbed many pagans and servants into their group, greatly increasing the size of the Israelite people.

After their journey into Egypt, their Exodus with the "mixed multitude" [non-Israelites who joined the nation as it left Egypt], and the receiving of the Torah on Mount Sinai, the Israelites returned to the land of Israel. Once again, they increased their numbers from among non-Israelite peoples, both those who lived in Canaan (such as the Hittites, Hivvites, Girgashites, Amorites, Perizzites, Jebusites, and others) and those who entered the land.

Some of these foreigners, the nokhri, remained apart from Israelite society, apart from the ezrach, the native Israelite. Some nokhri, though, wished to join the Israelites. Such people were given a new status, as gerim (Hebrew for "strangers"). A ger would be taken to the holy mountain and there render the necessary sacrifices.

Gerim often assimilated into the Israelite people by intermarriage. For instance, pagan women who married Jewish men automatically adopted their clan, and thus their religious views. The marriages that resulted were seen as positive because pagans would turn from idolatry to God through such marriages.

The gerim were permanent residents, but did not own land. All non-Israelites who joined a family or tribe were to be given equal rights and equal responsibilities, although the participation in religious rituals developed in stages. The Israelites were enjoined to love the gerim, for the Israelites had been gerim in Egypt.

As Judaism attracted adherents, it became both useful and necessary to explain the relationship between Jews and gentiles within Jewish thought. For a full theory of Jewish universalism [that is, a Jewish approach to conversion and proselytization] to develop, the central Jewish understanding of God had to undergo maturation.